What concept is used when a source document states "Operation Panda will take place on 29 September" and the new document states "On 29 September, Operation Panda will commence"?

The correct answer is based on the concept of "Revealed By." In this context, the original statement in the source document contains a specific piece of information — the date and designation of an operation, "Operation Panda." When that information is reiterated in a new document with slight wording changes but without introducing new information, it indicates that the new document is revealing information that is already present in the source document.

The language in both statements is essentially conveying the same actionable content regarding the operation's timing and identity — which is a classical example of revealing the same information without altering its essential nature or introducing additional sensitive data. Hence, this aligns with the defined concept of "Revealed By,” emphasizing that the new document does not add new classified data but simply reveals what was already stated.

Other concepts, such as "Derived By," pertain to processes that incorporate or synthesize existing information to create new understandings or interpretations. Concepts like "Extension" or "Modification" involve changing or expanding upon the details or context of the original material, which is not applicable here since the new document does not alter the nature of the information from the source but simply restates it in a different format.